Optical dispersion compensator on silicon

ABSTRACT

An optical dispersion compensator integrated with a silicon photonics system including a first phase-shifter coupled to a second phase-shifter in parallel on the silicon substrate characterized in an athermal condition. The dispersion compensator further includes a third phase-shifter on the silicon substrate to the first phase-shifter and the second phase-shifter through two 2×2 splitters to form an optical loop. A second entry port of a first 2×2 splitter is for coupling with an input fiber and a second exit port of a second 2×2 splitter is for coupling with an output fiber. The optical loop is characterized by a total phase delay tunable via each of the first phase-shifter, the second phase-shifter, and the third phase-shifter such that a normal dispersion (&gt;0) at a certain wavelength in the input fiber is substantially compensated and independent of temperature.

CROSS-REFERENCE TO RELATED APPLICATIONS

N/A

BACKGROUND OF THE INVENTION

The present invention relates to optical communication techniques. Moreparticularly, the present invention provides an optical dispersioncompensator integrated in a silicon photonics system.

Over the last few decades, the use of communication networks exploded.In the early days Internet, popular applications were limited to emails,bulletin board, and mostly informational and text-based web pagesurfing, and the amount of data transferred was usually relativelysmall. Today, Internet and mobile applications demand a huge amount ofbandwidth for transferring photo, video, music, and other multimediafiles. For example, a social network like Facebook processes more than500 TB of data daily. With such high demands on data and data transfer,existing data communication systems need to be improved to address theseneeds.

Progress in computer technology (and the continuation of Moore's Law) isbecoming increasingly dependent on faster data transfer between andwithin microchips. Optical interconnects may provide a way forward, andsilicon photonics may prove particularly useful, once integrated on thestandard silicon chips. 40-Gbit/s and then 100-Gbit/s data rates WDMoptical transmission over existing single-mode fiber is a target for thenext generation of fiber-optic communication networks. The big hangup sofar has been the fiber impairments like chromatic dispersion that areslowing the communication signal down. Chromatic dispersion is a resultof the dependence of the refractive index on the wavelength. Differentfrequency components of the light-wave experience different phase delaysdue to the refractive index change. The phase difference causesdistortion on the signal. Especially for high-speed communication beyond10 Gbits/s, distortion and attenuation of the optical signals take theirtoll.

In order to compensate the dispersion in the fiber, traditional methodis to use a discrete dispersion compensator formed on Silica-basedPlanar Lightwave Circuit (PLC). Such traditional device has very largedimension in centimeter range and is not suitable for small packagesilicon photonics modules. Therefore, an improved dispersion compensatorthat is compatible with a silicon photonics system is desired.

BRIEF SUMMARY OF THE INVENTION

The present invention relates to optical telecommunication techniques.More particularly, the present invention provides an optical dispersioncompensator that is based on an optical filter with small dimension andathermal characteristics directly integrated in a silicon photonics chipfor high data rate DWDM optical communications, though otherapplications are possible.

In modern electrical interconnect systems, high-speed serial links havereplaced parallel data buses, and serial link speed is rapidlyincreasing due to the evolution of CMOS technology. Internet bandwidthdoubles almost every two years following Moore's Law. But Moore's Law iscoming to an end in the next decade. Standard CMOS silicon transistorswill stop scaling around 5 nm. And the internet bandwidth increasing dueto process scaling will plateau. But Internet and mobile applicationscontinuously demand a huge amount of bandwidth for transferring photo,video, music, and other multimedia files. This disclosure describestechniques and methods to improve the communication bandwidth beyondMoore's law.

In an embodiment, the present invention provides an optical dispersioncompensator integrated with a silicon photonics system. The opticaldispersion compensator includes a first phase-shifter on a siliconsubstrate and a second phase-shifter on the silicon substrate.Additionally, the optical dispersion compensator includes a first 2×2splitter having a first exit port coupled to an input port of the firstphase-shifter and a second exit port coupled to an input port of thesecond phase-shifter and a second 2×2 splitter having a first entry portcoupled to an output port of the first phase-shifter and a second entryport coupled to an output port of the second phase-shifter. Furthermore,the optical dispersion compensator includes a third phase-shifter on thesilicon substrate having an input port coupled to a first exit port ofthe second 2×2 splitter and an output port coupled to a first entry portof the first 2×2 splitter to form an optical loop with the firstphase-shifter and the second phase-shifter. The second entry port of thefirst 2×2 splitter is for coupling with an input fiber and the secondexit port of the second 2×2 splitter is for coupling with an outputfiber. The optical loop is characterized by a total phase delay tunablevia each of the first phase-shifter, the second phase-shifter, and thethird phase-shifter such that a normal dispersion (>0) at a certainwavelength in the input fiber is substantially compensated andindependent of temperature.

In an alternative embodiment, the present invention provides a methodfor compensating fiber dispersion in a compact device integrated in asystem-on-chip. The method includes providing a silicon-on-insulatorsubstrate and forming a first waveguide and a second waveguide embeddedin a first cladding material on the silicon-on-insulator substrate. Thefirst waveguide and the second waveguide are optically coupled to eachother in parallel and respectively coupled to a first 2×2 coupler and asecond 2×2 coupler. Additionally, the method includes forming a windowof the first cladding material. Furthermore, the method includes formingthird waveguide in the window. The third waveguide is surrounded by asecond cladding material filled in the window. The third waveguide iscoupled to a first entry port of the first 2×2 coupler and a first exitport of the second 2×2 coupler to form an optical loop with the firstwaveguide and the second waveguide. Moreover, the method includescoupling a second entry port of the first 2×2 coupler to an input fiberand a second exit port of the second 2×2 coupler to an output fiber. Theoptical loop is characterized by a total phase delay tunable via each ofthe first waveguide, the second waveguide, and the third waveguide suchthat a normal dispersion (>0) at a certain wavelength in the input fiberis substantially compensated and independent of temperature.

In an alternative embodiment, the present invention provides asystem-on-chip for a silicon photonics system including a singlesilicon-on-insulator substrate formed with a dispersion compensatordescribed herein for compensating optical dispersion of a single-modefiber of a certain length at a certain wavelength of any channel in abroadband for telecommunication.

The present invention achieves these benefits and others in the contextof known silicon waveguide laser communication technology. However, afurther understanding of the nature and advantages of the presentinvention may be realized by reference to the latter portions of thespecification and attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The following diagrams are merely examples, which should not undulylimit the scope of the claims herein. One of ordinary skill in the artwould recognize many other variations, modifications, and alternatives.It is also understood that the examples and embodiments described hereinare for illustrative purposes only and that various modifications orchanges in light thereof will be suggested to persons skilled in the artand are to be included within the spirit and purview of this process andscope of the appended claims.

FIG. 1 is a plot of index of refraction for silica material versuswavelength.

FIG. 2 is a plot of group velocity dispersion of a typical single-modefiber versus wavelength.

FIG. 3 is a plot or relative delay time due to chromatic dispersion ofoptical fiber versus a relative change of pulse frequency of the opticalsignal.

FIG. 4 is a schematic diagram of an optical dispersion compensator basedon optical filters formed on silicon according to an embodiment of thepresent invention.

FIG. 5 is a result of a phase response of an optical dispersioncompensator according to an embodiment of the present invention.

FIG. 6 is a schematic diagram of the optical dispersion compensator inan athermal configuration according to a specific embodiment of thepresent invention.

FIG. 7 is a schematic diagram of the optical dispersion compensator inan athermal configuration according to another specific embodiment ofthe present invention.

FIG. 8 is flow chart showing a method for compensating fiber dispersionin a compact device integrated in a system-on-chip according to analternative embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to optical telecommunication techniques.More particularly, the present invention provides an optical dispersioncompensator that is based on optical filter formed directly on siliconsubstrate with small dimension and athermal characteristics and fullycompatible with silicon photonics system for high data rate DWDM opticalcommunications, though other applications are possible.

Group velocity dispersion (GVD) is the group delay (inverse of groupvelocity) dependence of an optical signal on the frequency orwavelength. In the silica material, which is a typical material foroptical fiber, as shown in FIG. 1, a shorter wavelength wave has highereffective refractive index and slower group velocity than a longerwavelength wave. When the shorter wavelength wave moves slower than thelonger ones in a fiber, the GVD is positive for this fiber, i.e., thefiber suffers a so-called normal GVD. If the GVD of a medium isnegative, the dispersion of this medium is an abnormal GVD. FIG. 2 showsa plot of optical dispersion (i.e., GVD) of a single-mode fiber atdifferent wavelength. As shown, for a wavelength of 1449 nm a normal GVDof ˜17 ps/nm/km is obtained for the fiber. In other words, a temporalbroadening of 1 ps is resulted for an optical signal pulse after it istransmitted through the optical fiber over 1 km in length, provided thatthe pulse width of the spectral line of a light source for transmittingthe signal is 1 nm.

As the optical signal travels a longer distance along the fiber, thefiber dispersion effect will be accumulated and becomes larger andlarger. The signal eventually will be distorted and lost along the way.Based on a dependency of relative time delay on relative frequency shiftshown in FIG. 3, when a light wave with a pulse width of 1 nm sent froma light source passes through a 5 km single-mode optical fiber with thenominal GVD of ˜17 ps/nm/km, the accumulated dispersion of the fiber atthe end of the 5 km will reach 849 ps/nm.

Optical filter in general has variable phase shift responses to theoptical waves it is transmitting. So it is possible to use an opticalfilter configured as a phase equalizer to couple to a single-mode fiberof a certain length for compensating the optical dispersion of thefiber. As the dispersion in the fiber is normal dispersion [e.g., 17ps/(nm×km)], so in order to compensate the fiber dispersion, an opticalfilter configured with an abnormal dispersion of a correspondingnegative value [e.g., −17 ps/(nm×km)] is needed.

The following description is presented to enable one of ordinary skillin the art to make and use the invention and to incorporate it in thecontext of particular applications. Various modifications, as well as avariety of uses in different applications will be readily apparent tothose skilled in the art, and the general principles defined herein maybe applied to a wide range of embodiments. Thus, the present inventionis not intended to be limited to the embodiments presented, but is to beaccorded the widest scope consistent with the principles and novelfeatures disclosed herein.

In the following detailed description, numerous specific details are setforth in order to provide a more thorough understanding of the presentinvention. However, it will be apparent to one skilled in the art thatthe present invention may be practiced without necessarily being limitedto these specific details. In other instances, well-known structures anddevices are shown in block diagram form, rather than in detail, in orderto avoid obscuring the present invention.

The reader's attention is directed to all papers and documents which arefiled concurrently with this specification and which are open to publicinspection with this specification, and the contents of all such papersand documents are incorporated herein by reference. All the featuresdisclosed in this specification, (including any accompanying claims,abstract, and drawings) may be replaced by alternative features servingthe same, equivalent or similar purpose, unless expressly statedotherwise. Thus, unless expressly stated otherwise, each featuredisclosed is one example only of a generic series of equivalent orsimilar features.

Furthermore, any element in a claim that does not explicitly state“means for” performing a specified function, or “step for” performing aspecific function, is not to be interpreted as a “means” or “step”clause as specified in 35 U.S.C. Section 112, Paragraph 6. Inparticular, the use of “step of” or “act of” in the Claims herein is notintended to invoke the provisions of 35 U.S.C. 112, Paragraph 6.

Please note, if used, the labels left, right, front, back, top, bottom,forward, reverse, entry, exit, clockwise and counter clockwise have beenused for convenience purposes only and are not intended to imply anyparticular fixed direction. Instead, they are used to reflect relativelocations and/or directions between various portions of an object.

FIG. 4 is a schematic diagram of an optical dispersion compensator basedon an optical filter formed on silicon substrate according to anembodiment of the present invention. This diagram is merely an example,which should not unduly limit the scope of the claims. One of ordinaryskill in the art would recognize many variations, alternatives, andmodifications. As shown, the optical filter 100 is provided as theoptical dispersion compensator formed on a silicon substrate,schematically shown in a cross-section view. Optionally, the siliconsubstrate is a silicon-on-insulator substrate. The optical dispersioncompensator 100 is a device fully integrated in a compact form in achip. In the embodiment, the optical dispersion compensator 100 includesa first phase-shifter 131 coupled a second phase-shifter 132 opticallyin parallel. At input side, a first 2×2 splitter 111 is coupled to aninput port of each of the first phase-shifter 131 and the secondphase-shifter 132. At output side, a second 2×2 splitter 112 is coupledto an output port of each of the first phase-shifter 131 and the secondphase-shifter 132. An entry port of the first 2×2 splitter 111 isconfigured to couple with an input fiber, which needs dispersioncompensation for an optical signal of a certain wavelength that hasalready been transported through the input fiber up to a certain length.An exit port of the second 2×2 splitter 112 is configured to couple withan output fiber for outputting the optical signal after the dispersioncompensation. Optionally, the 2×2 splitter is a multimode interferencecoupler. Optionally, the 2×2 splitter is a directional coupler.

Referring to FIG. 4, the optical dispersion compensator 100 furtherincludes a third phase-shifter 133 having an input port coupled toanother exit port of the second 2×2 splitter 112 and an output portcoupled to another entry port of the first 2×2 splitter 111 to form anoptical loop with the first phase-shifter 131 and the secondphase-shifter 132. Optionally, the optical loop is a feedback loop,which allows a portion of optical signal outputted at the output side togo through the third phase-shifter 133 and feedback to the input sideagain. This optical feedback loop effectively creates a path for tuningtotal phase delay to be associated with a negative dispersion requiredfor compensating normal dispersion of the optical signal of the certainwavelength caused by the optical fiber of the certain length.

Optionally the first phase-shifter 131 is a first waveguide formed onthe silicon substrate and the second phase-shifter 132 is a secondwaveguide formed on the same silicon substrate. Optionally, the secondwaveguide coupled with the first waveguide in parallel with a relativephase delay to form a Mach-Zehnder interferometer. Optionally, the thirdphase-shifter 133 is a third waveguide formed on the same siliconsubstrate.

Optionally, the first waveguide 131 comprises a first core material witha first index of refraction n₁ and an elongated shape of the firstlength L₁ embedded in a first cladding material 141 on the siliconsubstrate. In a cross-section view, an example of a waveguide is shownwith a core material in a typical rectangular shape embedded in acladding material overlying a substrate. The cladding material usuallyhas an index of refraction smaller than that of the core material sothat the light can be confined substantially inside the geometry of thecore of the waveguide. A combination effect of the waveguide core with acertain geometric shape and respective dimensions and the correspondingindices of refraction for both the first core material and the firstcladding material, a first phase delay for the optical signal of thecertain wavelength passing the first phase-shifter 131 is yielded.Similarly, the second waveguide 132 includes a second core material witha second index of refraction n₂ and an elongated shape of the secondlength L₂ embedded in the first cladding material 141 formed on the samesilicon substrate. A second phase delay for the optical signal of thecertain wavelength passing the second phase-shifter 132 is yielded. Thesecond phase delay may be different from the first phase delay.Optionally, the first waveguide and the second waveguide coupled inparallel with a relative phase delay to form a Mach-Zehnderinterferometer. Overall physical length of the Mach-Zehnderinterferometer including both 2×2 splitters 111 and 112 can be madequite compact. For example, the length of the dispersion compensator 100can be just about 100 μm.

In an embodiment, the third waveguide 133 includes a third core materialwith a third index of refraction n₃ and an elongated shape of the thirdlength L₃ embedded in a second cladding material 142. The secondcladding material 142 does not formed directly on the silicon substrate.Optionally, a window of the first cladding material 141 is created afterthe first cladding material is formed on the silicon substrate. Thesecond cladding material 142 is filled in the window of the firstcladding material 141. The third waveguide 133 is embedded within thesecond cladding material which is still chosen to have a smaller indexof refraction than the third index of refraction for confining lightinside the core material therein. Accordingly, the third phase delay forthe optical signal of the certain wavelength passing the thirdphase-shifter 133, which is set into an optical feedback loop, isyielded.

In an embodiment, a total phase delay of the optical dispersioncompensator 100 is a manifestation of the first phase delay, the secondphase delay, and the third phase delay associated with the structureprovided by the Mach-Zehnder interferometer formed by the firstwaveguide 131 and the second waveguide 132 and the optical feedback loopformed by coupling the third waveguide 133 to the Mach-Zehnderinterferometer. Each of the first waveguide, the second waveguide, andthe third waveguide can be independently fabricated and tuned withmaterial and geometry selections. Optionally, a heating element can beinstalled to be around each waveguide to tune the index of refraction bychanging temperature. As a result of tuning the first waveguide, thesecond waveguide, and the third waveguide as well as properly selectingthe corresponding first and second cladding materials under thestructure described herein, an abnormal dispersion (<0) for the certainwavelength can be achieved. The dispersion compensator 100 is directlyintegrated in a silicon chip with compact dimensions.

FIG. 5 is a result of a phase response of an optical dispersioncompensator according to an embodiment of the present invention. Thisdiagram is merely an example, which should not unduly limit the scope ofthe claims. One of ordinary skill in the art would recognize manyvariations, alternatives, and modifications. In an example, the fiberdispersion is assumed to be ˜17 ps/nm/km for a signal of a wavelength of˜1549.7 nm. A dispersion characteristics of a dispersion compensatorbased on the embodiment of the present disclosure is plotted. As shownin FIG. 5, in a wavelength range from 1549 nm to 1550 nm, a change ofphase delay curve 520 versus wavelength around 1549.7 nm yields anegative slope of −17 ps/nm/km in a bandwidth of 5 GHz, just perfect forcompensating the normal dispersion of the signal in the fiber. In thesame figure, a change of insertion loss of the signal versus wavelengthis also plotted, showing the insertion loss is well controlled below 0.5dB. At the specific wavelength 1549.7 nm, the insertion loss is only0.25 dB. The figure also shows a free-spectrum range (FSR) value of 50GHz is yielded, giving possibility for the same dispersion compensatorto compensate optical signal in different channel wavelengths in a broadband allowed by the FSR. Optionally, the dispersion compensator isdesigned with a single optimized signal wavelength. For compensatingmultiple signals with different wavelengths, usually multiple dispersioncompensator are separately employed, though at least certain numbers ofdifferent dispersion compensators for different wavelengths can be stillfabricated on a single silicon substrate, with feasibility to beintegrated in multi-channel DWDM silicon photonics system.

In some embodiments, tuning the first waveguide, the second waveguide,and the third waveguide can done both during their fabrication andafterward. Optionally, during the fabrication, the core material andcladding material of each waveguide can be properly selected forachieving different value of the phase delay. Optionally, the corematerial of each waveguide can be one selected from single crystalsilicon, poly-crystal silicon, SiN, Si₃N₄, SiON, silicon germanium alloy(Si_(x)Ge_(1-x)), GaAs, InP, InGaAsP, AlGaAs, Al₂O₃, LiNbO₃, and polymermaterial. Optionally, the core material for the first waveguide 131 isdifferent from that for the second waveguide 132. The first claddingmaterial 141 can be one selected from SiO₂, SiN, Si₃N₄, SiON, Air,silicon germanium alloy (Si_(x)Ge_(1-x)), and indium tin oxide.

In some embodiments, the core of each waveguide can be formed variablywith a length and cross-section shape. Optionally, the core can beformed with a cross-sectional shape in rectangular, a simple channelwaveguide. Or the core can be formed in complex shape such as acombination of two rectangular shapes. For example, a rib waveguidehaving a smaller rectangle on top of a wider rectangle, a slot-channelwaveguide having two rectangles in parallel separated by a small gap, aslot-rib waveguide having a slot-channel on top of a wider rectangle,and a multi-channel waveguide having two rectangles stacking together.In another example, a triangle shape waveguide can be provided.

In some embodiments, the optical dispersion compensator 100 is tunablein temperature for achieving certain value for proper dispersioncompensation and is an athermal dispersion compensator that isindependent from environmental temperature change. FIG. 6 is a schematicdiagram of the optical dispersion compensator 100 in an athermalconfiguration according to a specific embodiment of the presentinvention. Referring to FIG. 6, firstly, in the dispersion compensator100, each waveguide can be formed to have a heating element attached sothat its index of refraction can be tuned by changing the temperaturethereof. For example, a heating element 1311 is associated with thefirst waveguide 131. A heating element 1321 is attached with the secondwaveguide 132. Optionally, only one heating element is needed for theunit of the first waveguide 131 and the second waveguide 132. A heatingelement 1331 is disposed next to the third waveguide 133. This allowseach waveguide to be tunable after its fabrication on the siliconsubstrate has been done. By changing the index of refraction of eachwaveguide, a corresponding phase delay of the waveguide-basedphase-shifter is changed. Of course, the heating element, which istypically a resistor based device, has its own physical limit or tuningrange though it does not limit the claimed feature herein.

Secondly, in the dispersion compensator 100, two main units in theoptical loop, including the Mach-Zehnder interferometer formed by thefirst phase-shifter 131 and the second phase shifter 132, and the thirdphase-shifter 133 via the two 2×2 splitters, are constrainedrespectively in athermal conditions. Referring to FIG. 6, eachphase-shifter is a waveguide formed with a core of a certain shape and alength within a cladding, which can be characterized by an effectiveindex of refraction n_(eff) and a length L. The effective index ofrefraction of the phase-shifter depends on the shape and length of thecore, the indices of refraction of both the core material and thecladding material, and other properties such as optical-thermalcoefficient, mode structure associated with the geometry, wavelength andpolarization mode of signals, etc. For example, the first phase-shifter131 is characteristically marked with (n_(eff1), L₁). Similarly, thesecond phase-shifter 132 is marked with (n_(eff2), L₂) and the thirdphase-shifter 133 is marked with (n_(eff3), L₃). For the Mach-Zehnderinterferometer formed by the first phase-shifter 131 and the secondphase shifter 132, the athermal condition is to ensure that atemperature variation of a first effective index of refraction n_(eff1)multiplied by a first length L₁ of the first phase-shifter cancels atemperature variation of a second effective index of refraction n_(eff2)multiplied by a second length L₂ of the second phase-shifter. This isrepresented by an equation (1) below:

$\begin{matrix}{{{\frac{{dn}_{{eff}\; 1}}{d\; T}L_{1}} - {\frac{{dn}_{{eff}\; 2}}{d\; T}L_{2}}} = 0} & (1)\end{matrix}$For the third phase-shifter 133, in some embodiment, it is set under anathermal condition by itself such that its effective index of refractionis temperature independent. This is represented by another equation (2):

$\begin{matrix}{\frac{{dn}_{{eff}\; 3}}{d\; T} = 0} & (2)\end{matrix}$Practically, the second cladding material used to surround the thirdwaveguide core needs to be properly selected. Optionally, the core ofthe third waveguide 133 can be one material selected from single crystalsilicon, poly-crystal silicon, SiN, Si₃N₄, SiON, silicon germanium alloy(Si_(x)Ge_(1-x)), GaAs, InP, InGaAsP, AlGaAs, Al₂O₃, LiNbO₃, and polymermaterial, similar to that for forming the first or second waveguidecores. However, the second cladding material needs to be selected with athermal optical coefficient in an opposite sign compared to that of thethird waveguide core. For example, if a core material of silicon is usedto form the third waveguide which has a positive thermal-opticalcoefficient of +1.84×10⁻⁴/K (as other typical material for waveguidecore), the cladding material must have a negative thermal-opticalcoefficient. Optionally, one such material selected from PolymethylMethacrylate (PMMA) and Potassium Aluminophosphate glass (KAP) is usedfor forming the second cladding material 142 which is filled into thewindow in the first cladding material 141. PMMA has a thermal-opticalcoefficient of −1.1×10⁻⁴/K. KAP has a thermal-optical coefficient of−0.92×10⁻⁴/K.

In general, the dispersion compensator provided in the presentdisclosure can be duplicated as a whole and cascaded in series multipletimes to have an accumulated dispersion for compensating largerdispersion of a longer input fiber. Multiple different dispersioncompensators tuned for different wavelengths can be separatelyintegrated in a multi-channel silicon photonics system.

In some embodiments, a unit of the first phase-shifter 131 and thesecond phase-shifter 132 including at least the first 2×2 splitter 111can be used as a duplicate phase-shifter unit. The duplicatephase-shifter unit can be cascaded in series multiple times in lowerbranch of the optical loop that couples with the third phase-shifter 133to form a different dispersion compensator for achieving differentvalues in total phase delay. Optionally, each of those duplicatephase-shifter units is constrained under the athermal condition that atemperature variation of a first effective index of refractionmultiplied by the first length of the first phase-shifter thereincancels a temperature variation of a second effective index ofrefraction multiplied by the second length of the second phase-shiftertherein.

In some embodiments, the third phase-shifter includes at least twowaveguides configured couple to each other in parallel. FIG. 7 shows anexample of the embodiments of the optical dispersion compensator of thepresent disclosure. As shown, the optical dispersion compensator 200 hasa first phase-shifter 231 and a second phase-shifter 232 configured thesame way as those in the optical dispersion compensator 100 using afirst 2×2 splitter 211 at the input side and a second 2×2 splitter 212at the output side. The third phase-shifter 133 in the opticaldispersion compensator 100 is replaced by a unit of a thirdphase-shifter 271 and a fourth phase shifter 272 coupled to each otherin parallel. The coupled third phase-shifter 271 and the fourth phaseshifter 272 still forms an optical loop via a first 1×2 coupler 251 tocouple with an entry port of the first 2×2 splitter 211 and a second 2×1coupler 252 to couple with an exit port of the second 2×2 splitter.

Optionally, every phase-shifter in the optical dispersion compensator200 is a waveguide formed on a same silicon substrate. Optionally, thefirst phase-shifter 231 is a first waveguide surrounded by a firstcladding 241 on the silicon substrate characterized by two keyparameters i.e., a first effective index of refraction n_(eff1) and afirst length L₁, and the second phase-shifter 232 is a second waveguidesurrounded by the same first cladding 241 on the silicon substratecharacterized by a second effective index of refraction n_(eff2) and asecond length L₂. Optionally, the unit of the first phase-shifter andthe second phase-shifter coupled in parallel is configured to be anathermal unit, satisfying the condition that a temperature variation ofthe first effective index of refraction n_(eff1) multiplied by the firstlength L₁ of the first phase-shifter cancels a temperature variation ofthe second effective index of refraction n_(eff2) multiplied by thesecond length L₂ of the second phase-shifter.

Optionally, the third phase-shifter 271 is a third waveguide formed in awindow of the first cladding material 241 filled by a second claddingmaterial 242. Optionally, the fourth phase-shifter 272 is a duplicatewaveguide the same as the third phase-shifter 271 having substantiallythe same structure and optical-thermal properties and embedded in thesame (second) cladding 242. The unit of the two duplicated waveguidesensures that it is automatically an athermal unit. In anotherembodiment, the fourth phase-shifter 272, characterized similarly by twokey parameters (n_(eff4), L₄), is a different waveguide yet embedded thesame (second cladding as the third phase-shifter 271 characterized by(n_(eff3), L₃). Optionally, a proper selection of core materials andgeometries of the two waveguides in the second cladding can still ensurethat the unit of the third phase-shifter and the fourth phase-shiftercoupled in parallel is an athermal unit, i.e., a temperature variationof the effective index of refraction n_(eff3) multiplied by the lengthL₃ of the third phase-shifter 271 cancels a temperature variation of theeffective index of refraction n_(eff4) multiplied by the length L₄ ofthe fourth phase-shifter 272. Yet, the difference of the unit of thethird phase-shifter and the fourth phase-shifter coupled in parallel inthe dispersion compensator 20 versus the third phase-shifter 133 in thedispersion compensator 100 will likely result different phase delay anddifferent effect of temperature tuning if a heating element is installedin each unit.

In an alternative embodiment, the unit of the third phase-shifter andthe fourth phase-shifter coupled in parallel can be used as a duplicateunit and cascaded in parallel multiple times to achieve different valuesof total phase delay of the optical dispersion compensator. Yet, as moreunits are cascaded, each of all units are coupled to each previous unitincluding the unit of the first phase-shifter and the secondphase-shifter using two 2×2 couplers (like 211 and 212) except that thelast unit is coupled to the second-to-the-last unit using two 1×2couplers (like 251 and 252).

In another aspect, the present disclosure provides a method forcompensating dispersions in optical fiber in a compact device that isintegrated in a system-on-chip. FIG. 8 is flow chart showing a methodfor compensating fiber dispersion in a compact device integrated in asystem-on-chip according to an alternative embodiment of the presentinvention. This diagram is merely an example, which should not undulylimit the scope of the claims. One of ordinary skill in the art wouldrecognize many variations, alternatives, and modifications. Referring toFIG. 8, the method 800 includes a process 810 of providing asilicon-on-insulator substrate. This substrate has been used forintegrating various kinds of silicon photonics systems including DWDMmodule, transceiver module, opto-electric control module, polarizationconverter, photodetector, etc. and can be used for integrating one ormore dispersion compensator in the same substrate.

In the embodiment, the method 800 includes a process 820 for forming afirst waveguide and a second waveguide coupled in parallel and embeddedin a first cladding material on the silicon-on-insulator substrate. Inthe process, each of the first waveguide and the second waveguide canhave their input ports coupled to a first 2×2 splitter at an input side.One entry port of the 2×2 splitter is able to couple directly to aninput fiber to receive the optical signal that has accumulateddispersion after being transported through a certain substantial lengthof the input fiber. The two output ports of the first waveguide and thesecond waveguide are coupled to a second 2×2 splitter at an output sidewith one exit port being coupled to an output fiber (configured tooutput the optical signal after dispersion compensation).

Optionally, the process of forming the first waveguide and the secondwaveguide includes forming at least a heating element thereof configuredto tune phase delays of the first waveguide and the second waveguideindependently.

Optionally, the process of forming the first waveguide and the secondwaveguide further comprises selecting proper core materials and lengthsrespectively for the first waveguide and the second waveguide andcladding material for the first cladding material such that acombination of the first waveguide and the second waveguide isconstrained to an athermal condition that a temperature variation of afirst effective index of refraction multiplied by a first length of thefirst waveguide cancels a temperature variation of a second effectiveindex of refraction multiplied by a second length of the waveguide. As aresult, the unit of the first waveguide and the second waveguide coupledin parallel forms an athermal Mach-Zehnder Interferometer withrespective tunable phase delay.

In the embodiment, the method 800 further includes a process 830 forforming a window in the first cladding material on thesilicon-on-insulator substrate. Optionally, the first cladding materialis silicon dioxide. Furthermore, the method includes a process 840 forforming a third waveguide surrounded by a second cladding material thatis filled into the window. Optionally, the third waveguide is configuredto coupled respectively to another entry port of the first 2×2 splitterand another exit port of the second 2×2 splitter to form an optical loopwith the first waveguide and the second waveguide. The optical loop is afeedback loop to allow partial signal out of the second 2×2 splitter toreturn through the third waveguide to the input side. The optical loopstructure provides a basis of manifestation of the three waveguides toyield an abnormal dispersion (<0) that is able to compensate the normaldispersion caused by a certain length of single-mode fiber.

Accordingly, the method 800 includes a process 850 for tuning the first,second, and third waveguide and first, second cladding in the opticalloop to achieve a total phase delay that yields an abnormal dispersionto compensate a normal dispersion of an input fiber and beingtemperature independent. Optionally, the second cladding material ischosen to have a thermal-optical coefficient with an opposite sign ofthat of the core material of the third waveguide. Therefore, the thirdwaveguide itself can be configured to be an athermal phase-shifter thatkeeps the temperature variation of its effective index of refractionsubstantially at zero. Since the unit of the first waveguide and thesecond waveguide coupled in parallel also forms an athermal Mach-ZehnderInterferometer. The optical dispersion compensator formed above is ableto compensate the fiber dispersion at a certain wavelength substantiallytemperature independent.

Optionally, the method of forming the third waveguide further includesforming a combined phase-shifter including two waveguides coupled toeach other in parallel. The two waveguides are respectively coupled withthe first entry port of the first 2×2 splitter via a first 1×2 splitterand the first exit port of the second 2×2 splitter via a second 2×1splitter to form an optical loop. The two waveguides are either twoidentical ones or constrained under an athermal condition that atemperature variation of a first effective index of refractionmultiplied by a first length of a first one of the two waveguidestherein cancels a temperature variation of a second effective index ofrefraction multiplied by a second length of a second one of the twowaveguides therein.

Optionally, the method also includes cascading in series multiple unitsof the first waveguide and the second waveguide in the optical loop withthe third waveguide for achieving different values of total phase delay.Each unit of the first waveguide and the second waveguide is coupled inparallel to each other and still satisfied the athermal condition.

Optionally, the method also includes cascading in parallel multipleunits of the combined phase-shifter in the optical loop with the firstwaveguide and the second waveguide for achieving different values oftotal phase delay. Each unit of the combined phase-shifter isconstrained under the athermal condition.

In yet another aspect, the present disclosure provides a siliconphotonics system comprising a single silicon-on-insulator substrateformed with a dispersion compensator described herein.

While the above is a full description of the specific embodiments,various modifications, alternative constructions and equivalents may beused. Therefore, the above description and illustrations should not betaken as limiting the scope of the present invention which is defined bythe appended claims.

What is claimed is:
 1. An optical dispersion compensator integrated witha silicon photonics system comprising: a first phase-shifter on asilicon substrate; a second phase-shifter on the silicon substrate; afirst 2×2 splitter having a first exit port coupled to an input port ofthe first phase-shifter and a second exit port coupled to an input portof the second phase-shifter; a second 2×2 splitter having a first entryport coupled to an output port of the first phase-shifter and a secondentry port coupled to an output port of the second phase-shifter; athird phase-shifter on the silicon substrate having an input portcoupled to a first exit port of the second 2×2 splitter and an outputport coupled to a first entry port of the first 2×2 splitter to form anoptical loop with the first phase-shifter and the second phase-shifter;wherein the second entry port of the first 2×2 splitter is for couplingwith an input fiber and the second exit port of the second 2×2 splitteris for coupling with an output fiber, wherein the optical loop ischaracterized by a total phase delay tunable via each of the firstphase-shifter, the second phase-shifter, and the third phase-shiftersuch that a normal dispersion (>0) at a certain wavelength in the inputfiber is substantially compensated and independent of temperature. 2.The optical dispersion compensator of claim 1, wherein the firstphase-shifter is a first waveguide formed on the silicon substrate, thesecond phase-shifter is a second waveguide formed on the same siliconsubstrate, the second waveguide coupled with the first waveguide inparallel to form a Mach-Zehnder interferometer, and the thirdphase-shifter is a third waveguide formed on the same silicon substrate.3. The optical dispersion compensator of claim 2, wherein the firstwaveguide comprises a first core material with a first index ofrefraction and an elongated shape of the first length embedded in afirst cladding material on the silicon substrate yielding a first phasedelay at the certain wavelength, the second waveguide comprises a secondcore material with a second index of refraction and an elongated shapeof the second length embedded in the first cladding material on thesilicon substrate yielding a second phase delay at the certainwavelength, the third waveguide comprises a third core material with athird index of refraction and an elongated shape of the third lengthembedded in a second cladding material filled in a window of the firstcladding material on the silicon substrate yielding a third phase delayat the certain wavelength.
 4. The optical dispersion compensator ofclaim 3, wherein the total phase delay is a manifestation of the firstphase delay, the second phase delay, and the third phase delayassociated with the Mach-Zehnder interferometer and the optical loop toachieve an abnormal dispersion (<0) that just compensates the normaldispersion at the certain wavelength in the input fiber.
 5. The opticaldispersion compensator of claim 2, wherein the silicon substrate is asilicon-on-insulator substrate used for forming a chip of the siliconphotonics system.
 6. The optical dispersion compensator of claim 3,wherein the first core material and the second core material are atleast one material selected from single crystal silicon, poly-crystalsilicon, SiN, Si₃N₄, SiON, silicon germanium alloy (Si_(x)Ge_(1-x)),GaAs, InP, InGaAsP, AlGaAs, Al₂O₃, LiNbO₃, and polymer material, and thethird core material comprises a material selected from single crystalsilicon, poly-crystal silicon, SiN, Si₃N₄, SiON, silicon germanium alloy(Si_(x)Ge_(1-x)), GaAs, InP, InGaAsP, AlGaAs, Al₂O₃, LiNbO₃, and polymermaterial.
 7. The optical dispersion compensator of claim 6, wherein thefirst cladding material comprises a material having an index ofrefraction smaller than each of the first index of refraction and thesecond index of refraction, selected from SiO₂, SiN, Si₃N₄, SiON, Air,silicon germanium alloy (Si_(x)Ge_(1-x)), and indium tin oxide, thesecond cladding material comprises a material having an index ofrefraction smaller than the third index of refraction and a negativethermal-optical coefficient, selected from Polymethyl Methacrylate(PMMA) and Potassium Aluminophosphate glass (KAP).
 8. The opticaldispersion compensator of claim 3, wherein each of the first corematerial, the second core material, and the third core materialcomprises a cross-section shape selected from a rectangle, a combinationof two rectangles, and a triangle.
 9. The optical dispersion compensatorof claim 3, wherein the first phase-shifter, the second phase-shifter,and the third phase-shifter comprises respectively a first heatingelement, a second heating element, and a third heating element forindependently tuning the first index of refraction, the second index ofrefraction, and the third index of refraction by changing temperaturethereof.
 10. The optical dispersion compensator of claim 9, wherein thefirst phase-shifter and the second phase-shifter are substantiallylimited under an athermal condition that a temperature variation of afirst effective index of refraction multiplied by the first lengthcancels a temperature variation of a second effective index ofrefraction multiplied by the second length, wherein the first effectiveindex of refraction is resulted from both the first core material andthe first cladding material and the second effective index of refractionis resulted from both the second core material and the first claddingmaterial; the third phase-shifter is configured to ensure that atemperature variation of a third effective index of refraction resultedfrom both the third core material and the second cladding material issubstantially zero.
 11. The optical dispersion compensator of claim 1,wherein the input fiber is a single-mode fiber.
 12. The opticaldispersion compensator of claim 1, wherein the certain wavelength can beat least one wavelength corresponding to one channel selected from abroad band for telecommunication.
 13. The optical dispersion compensatorof claim 1, wherein each of the first 2×2 splitter and the second 2×2splitter is one selected from a multimode-interference (MMI) coupler anda directional coupler.
 14. The optical dispersion compensator of claim1, wherein the first phase-shifter and the second phase-shifterincluding at least the first 2×2 splitter can be used as a duplicatephase-shifter unit cascaded in series multiple times in the optical loopthat couples with the third phase-shifter for achieving different valuesin total phase delay, wherein each phase-shifter unit is constrainedunder the athermal condition that a temperature variation of a firsteffective index of refraction multiplied by the first length of thefirst phase-shifter therein cancels a temperature variation of a secondeffective index of refraction multiplied by the second length of thesecond phase-shifter therein.
 15. The optical dispersion compensator ofclaim 1, wherein the third phase-shifter is a combined phase-shifterincluding two waveguides coupled to each other in parallel, the twowaveguides being respectively coupled with the first entry port of thefirst 2×2 splitter via a first 1×2 splitter and the first exit port ofthe second 2×2 splitter via a second 2×1 splitter to form the opticalloop, wherein the two waveguides are either two identical ones orconstrained under an athermal condition that a temperature variation ofa first effective index of refraction multiplied by a first length of afirst one of the two waveguides therein cancels a temperature variationof a second effective index of refraction multiplied by a second lengthof a second one of the two waveguides therein.
 16. The opticaldispersion compensator of claim 15, wherein the third phase-shifter canbe further used as a duplicate phase-shifter unit cascaded in parallelmultiple times in the optical loop associated with the firstphase-shifter and the second phase-shifter for achieving differentvalues of total phase delay.
 17. A method for compensating fiberdispersion in a compact device integrated in a system-on-chip,comprising: providing a silicon-on-insulator substrate; forming a firstwaveguide and a second waveguide embedded in a first cladding materialon the silicon-on-insulator substrate, the first waveguide and thesecond waveguide optically coupled to each other in parallel andrespectively coupled to a first 2×2 coupler and a second 2×2 coupler;forming a window of the first cladding material; forming third waveguidein the window, the third waveguide being surrounded by a second claddingmaterial filled in the window, the third waveguide being coupled to afirst entry port of the first 2×2 coupler and a first exit port of thesecond 2×2 coupler to form an optical loop with the first waveguide andthe second waveguide; coupling a second entry port of the first 2×2coupler to an input fiber and a second exit port of the second 2×2coupler to an output fiber; wherein the optical loop is characterized bya total phase delay tunable via each of the first waveguide, the secondwaveguide, and the third waveguide such that a normal dispersion (>0) ata certain wavelength in the input fiber is substantially compensated andindependent of temperature.
 18. The method of claim 17, wherein formingthe first waveguide and the second waveguide comprises forming at leasta heating element configured to tune phase delays of the first waveguideand the second waveguide.
 19. The method of claim 17, wherein formingthe first waveguide and the second waveguide further comprises selectingproper core materials and lengths respectively for the first waveguideand the second waveguide and cladding material for the first claddingmaterial such that a combination of the first waveguide and the secondwaveguide is constrained to an athermal condition that a temperaturevariation of a first effective index of refraction multiplied by a firstlength of the first waveguide cancels a temperature variation of asecond effective index of refraction multiplied by a second length ofthe waveguide.
 20. The method of claim 17, wherein forming the thirdwaveguide comprises selecting proper core material and length for thethird waveguide and cladding material for the second cladding materialsuch that the third waveguide is constrained to an athermal conditionthat a temperature variation of an effective index of refraction issubstantially zero.
 21. The method of claim 17, wherein forming thethird waveguide further comprises forming a combined phase-shifterincluding two waveguides coupled to each other in parallel, the twowaveguides being respectively coupled with the first entry port of thefirst 2×2 splitter via a first 1×2 splitter and the first exit port ofthe second 2×2 splitter via a second 2×1 splitter to form the opticalloop, wherein the two waveguides are either two identical ones orconstrained under an athermal condition that a temperature variation ofa first effective index of refraction multiplied by a first length of afirst one of the two waveguides therein cancels a temperature variationof a second effective index of refraction multiplied by a second lengthof a second one of the two waveguides therein.
 22. The method of claim19, further comprising cascading in series multiple units of the firstwaveguide and the second waveguide in the optical loop with the thirdwaveguide for achieving different values of total phase delay, each unitof the first waveguide and the second waveguide being coupled inparallel to each other and still satisfied the athermal condition. 23.The method of claim 21, further comprising cascading in parallelmultiple units of the combined phase-shifter in the optical loop withthe first waveguide and the second waveguide for achieving differentvalues of total phase delay, wherein each unit of the combinedphase-shifter is constrained under the athermal condition.
 24. Asystem-on-chip for a silicon photonics system comprising a singlesilicon-on-insulator substrate formed with a dispersion compensator ofclaim 1.